U.S. Energy Secretary Steven Chu's Latest Experiment

Chu's atom interferometer could lead to GPS without the satellites

22 January 2009—The newly confirmed U.S. Secretary of Energy, Steven Chu, is settling into his new job in Washington, D.C., but he’s still making waves in his old job as a physicist. The former director of the Lawrence Berkeley National Laboratory, Chu presents in his latest paper one of the most promising developments yet in a fledgling quantum technology that may within a decade power satellite-free GPS, monitor earthquake zones, map out undiscovered mineral resources, and search for elusive gravitational waves.

The technology is based on the 85-year-old principle that particles of matter act like waves. Chu’s team, directed by physics assistant professor Holger Müller, of the University of California, Berkeley, nudged individual cesium atoms up a spout and watched the resulting patterns as the atoms fell onto a detector below.

Like a water fountain, the atoms’ rippling patterns tell us something about their paths. Choppy waves at the detector indicate sputtering spouts of cesium, whereas smooth sine-wave patterns reveal a steadier fountain. However, unlike a water fountain, the waves in the Chu group’s atom interferometers originate in each individual cesium atom. Each atom interferes with itself to form the pattern on the detector.

”Somehow you need to split the matter wave and [then] recombine it,” says Müller. ”The waves may be in phase [at the detector], and that means you count lots of atoms. Or the waves will cancel, in which case you find no atoms at all.”

The cesium atoms in Chu and Müller’s device start off trapped at the intersection of criss-crossed laser beams, confined by a magnetic field and at a temperature near absolute zero. (Chu shared the physics Nobel Prize in 1997 for developing such laser atom traps.) The magnetic field is then turned off, and the atoms slowly drift upward.

As the atoms drift, they are also tweaked by infrared laser pulses from below. The intensity of the pulses is just shy of being enough to provide extra upward kicks. At such delicate laser intensities, the laws of quantum mechanics step in, essentially splitting each cesium atom into two alternate realities. In one, the laser can push the atom up. In the other, the atom simply floats, unmoved by the light from below.

Each cesium atom travels both paths simultaneously, rising as high as a meter above the atom trap, Müller says. But whether boosted by laser pulses or floating up from the release of the trap, the atoms are ultimately pulled back to Earth by gravity. As the atoms strike the detector, each atom’s two alternate paths are forced to recombine, providing the same kinds of rich interference patterns that converging beams of light trace in the making of holograms.

The difference, Müller says, is that atoms feel the pull of gravity, while light feels almost none. ”That makes an atom interferometer much more sensitive to gravitational effects than a [laser] interferometer,” he says.

Mark Kasevich, professor of physics at Stanford University, is one of the world’s leading experts in developing applications for atom interferometers. Kasevich, a former graduate student of Chu’s, says that Chu and Müller have created a system that has given the ”best ever result” for the next-generation of atom interferometers. ”This is clearly the future,” Kasevich says of the new research.

Kasevich says his lab is developing geological applications for atom interferometers that he expects to see in five years or less. Such applications include using differences in the strength of gravity to detect underground water tables, diamond deposits, or slight motions of tectonic plates that could presage an earthquake.

Perhaps in 10 years, Kasevich adds, these atom interferometers could lead to breakthroughs in navigation technologies that don’t need satellites. Instead, a series of microscopic interferometers would infer the position from precision measurements of accelerations and decelerations. Kasevich is also researching atom interferometers as the basis for ”telescopes” that could detect gravitational waves from orbiting pairs of black holes in nearby galaxies or see the restless, rumbling dawn of the early universe.

Müller says he’s now finishing a few other papers with Chu from research done before the Energy Secretary’s appointment to President Obama’s cabinet. Beyond that, though, Müller says he expects Chu to set aside the atom interferometer research for now and concentrate on his primary passion: powering a productive planet with as little fossil fuel as possible.

”[Chu] is very ambitious and always very energetic,” Müller says.

”Anytime Steve takes a step in a new direction, he succeeds,” Kasevich adds, ”in remarkable ways.”

About the Author

MARK ANDERSON is a freelance science and technology writer based in Northampton, Mass. In October 2008, he wrote about the technological rescue and financial bailout of the Gravity Probe B experiment.

To Probe Further

Chu’s paper, which has been submitted to the journal Physical Review Letters , can be found at the physics abstract repository arxiv.org.